Current aperture vertical electron transistors with ammonia molecular beam epitaxy grown P-type gallium nitride as a current blocking layer

ABSTRACT

A current aperture vertical electron transistor (CAVET) with ammonia (NH 3 ) based molecular beam epitaxy (MBE) grown p-type Gallium Nitride (p-GaN) as a current blocking layer (CBL). Specifically, the CAVET features an active buried Magnesium (Mg) doped GaN layer for current blocking purposes. This structure is very advantageous for high power switching applications and for any device that requires a buried active p-GaN layer for its functionality.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. Section 119(e) ofthe following and commonly-assigned U.S. Provisional PatentApplications:

U.S. Provisional Patent Application Ser. No. 61/499,076, filed on Jun.20, 2011, by Srabanti Chowdhury, Ramya Yeluri, Christophe Hurni, UmeshK. Mishra, and Ilan Ben-Yaacov, entitled “CURRENT APERTURE VERTICALELECTRON TRANSISTORS WITH AMMONIA MOLECULAR BEAM EPITAXY GROWN P-TYPEGALLIUM NITRIDE AS A CURRENT BLOCKING LAYER”; and

U.S. Provisional Patent Application Ser. No. 61/583,015, filed on Jan.4, 2012, by Srabanti Chowdhury, Ramya Yeluri, Christophe Hurni, Umesh K.Mishra, and Ilan Ben-Yaacov, entitled “CURRENT APERTURE VERTICALELECTRON TRANSISTORS WITH AMMONIA MOLECULAR BEAM EPITAXY GROWN P-TYPEGALLIUM NITRIDE AS A CURRENT BLOCKING LAYER”,

both of which applications are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is related generally to the field of electronic devices,and more particularly, to current aperture vertical electron transistors(CAVETs) with ammonia (NH₃) molecular beam epitaxy (MBE) grown p-typeGallium Nitride (GaN) as a current blocking layer (CBL).

2. Description of the Related Art

(Note: This application references a number of different publications asindicated throughout the specification by one or more reference numberswithin brackets, e.g., [x]. A list of these different publicationsordered according to these reference numbers can be found below in thesection entitled “References.” Each of these publications isincorporated by reference herein.)

FIG. 1 is a schematic illustration of a CAVET 100, includinghigher/heavily n-type doped Gallium Nitride (n⁺-GaN) 102, lower orlightly n-type doped GaN (n⁻-GaN) 104, aperture 106, Current BlockingLayer (CBL), unintentionally doped (UID) GaN 108, aluminum galliumnitride (AlGaN) 110, source 112, gate 114, and drain 116. The CAVET 100is a vertical device comprised of an n-type doped drift region 118 tohold voltage and a horizontal two-dimensional electron gas (2DEG) 120 tocarry current flowing from the source 112, under a planar gate 114, andthen in a vertical direction to the drain 116 through an aperture 106.

As shown in FIG. 1, electrons flow horizontally from the source 112through the 2DEG (dashed lines 120), and then vertically through theaperture region 106 to the drain 116, and are modulated by the gate 114.A fundamental part of a CAVET is the CBL, which blocks the flow of thecurrent and causes on-state current to flow through the aperture 106.

Previously, the CBL has been fabricated by ion implantation. Forexample, two prior art designs of a CBL in a CAVET are described below:

1. AlGaN/GaN CAVETs with Aluminum (Al) ion implanted GaN as the CBL [1];and

2. AlGaN/GaN CAVETs with Magnesium (Mg) ion implanted GaN as the CBL[2].

In both prior art designs, functioning devices have been achieved bysuccessfully blocking the current from flowing through the CBL region bythe use of an ion-implanted GaN layer as a CBL region. The damaged(trap-filled) CBL region introduced a barrier to the electrons injectedfrom the source, thereby preventing the electrons from flowing directlyinto the drain without passing under the gate.

Nonetheless, there is a need in the art for improvements in CAVETdesigns. The present invention satisfies that need.

SUMMARY OF THE INVENTION

To overcome the limitations in the prior art described above, and toovercome other limitations that will become apparent upon reading andunderstanding the present specification, the present invention disclosesa CAVET including an aperture region in a III-nitride current blockinglayer, wherein a barrier to electron flow through the III-nitride CBL isat least 1 or 2 electron-Volt(s).

The III-nitride CBL can be an active p-type doped III-nitride layer,such as an active p-type GaN layer or an active Magnesium doped layer.The aperture region can comprise n-type GaN.

The CBL can cause on-state current to flow through the aperture region.

The CBL's thickness (e.g., at least 10 nanometers (nm)), holeconcentration, and composition, can be such that the barrier to electronflow has the desired value (e.g., at least 1 electron volt).

The device can further comprise an active region comprising a twodimensional electron gas confined in a GaN layer by an AlGaN barrierlayer; a source contact to the GaN layer and the AlGaN barrier layer; adrift region, comprising one or more n-type GaN layers, wherein the CBLis between the drift region and the active region; a drain contact tothe drift region, and a gate positioned on or above the active regionand the aperture, to modulate a current between the source and thedrain.

The n-type III-nitride drift region can be between the aperture regionand the drain. An n-type doping concentration in the drift region can beless than an n-type doping concentration in the aperture region.

The source and the CBL can be electrically connected such that inoperation there is no bias between the source and the CBL.

The present invention further discloses a III-nitride CAVET, comprisinga current blocking layer, wherein the CBL is such that the CAVET isoperable to prevent a current density of greater than 0.4 A/cm² fromflowing through the CBL when the CAVET is biased in an off state with asource-drain voltage of about 400V or 400 V or less.

The present invention further discloses a method of fabricating anelectronic device, comprising defining an aperture region and asacrificial region in a first III-nitride layer; removing the firstIII-nitride layer in the sacrificial region; forming the III-nitride CBLaround the aperture region, and forming one or more second III-Nitridelayers on both the first III-Nitride layer and the III-Nitride currentblocking layer. A mask can be formed over the aperture region prior toremoving the first III-nitride layer in the sacrificial region. The maskcan be removed prior to forming the second III-Nitride layers.

The CBL can be grown using ammonia (NH₃) based molecular beam epitaxy(MBE).

The CBL can be grown by a Metal Organic Chemical Vapor Deposition(MOCVD) growth technique by doping a Gallium Nitride layer with Mgdopants, wherein the III-nitride current blocking layer is activated byannealing in a hydrogen free environment at a temperature above 700° C.to make the III-nitride CBL a p-type III-nitride current blocking layer.The second III-nitride layers comprising AlGaN/GaN layers can then beregrown in ammonia-MBE which does not passivate the Mg acceptors,thereby preserving the p-type behavior of the III-nitride CBL.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a cross-sectional schematic illustration of a CAVET.

FIGS. 2( a)-(i) are cross-sectional schematic illustrations of the stepsused to fabricate a CAVET according to one embodiment of the presentinvention.

FIG. 3 plots the band structure of the CAVET fabricated according to themethod of FIG. 2( a)-(i).

FIG. 4( a) plots Mg concentration vs. estimated depth through the CAVETstructure and measured by Secondary Ion Mass Spectroscopy (SIMS).

FIG. 4( b) plots current density as a function of drain-source voltage(V_(ds)) for a CAVET with a zero aperture and active p-GaN CBL.

FIG. 5( a) is a cross-sectional schematic of a structure for measuringthe CBL blocking capacity of the p-GaN CBL in the CAVET.

FIG. 5( b) is a graph of the forward bias current-voltage (I-V)characteristics of the structure in FIG. 5( a), and FIG. 5( c) is agraph of the reverse bias I-V characteristics of the structure in FIG.5( a).

FIG. 5( d) is a graph of the I-V characteristics of a structure similarto that shown in FIG. 5( a), reflecting the I-V characteristics of theCAVET having the structure of FIG. 2( i), plotting current (Amps, A) andcurrent density (A/cm²) as a function of voltage, wherein the back p-ndiode has a leakage current of 140 μA or 0.35 A/cm² at a reverse biasvoltage of 400V.

FIG. 6 is a graph of the direct current (DC) I-V characteristics of theCAVET, plotting drain-source current (I_(ds)) as a function of V_(ds)for a device with aperture length L_(ap)=15 μm, 30 μm×75 μm device area,and wherein each curve is for a different gate source voltage (Vgs), asVgs is ramped from 0 V to −10 V (Vgs=0 V, −2 V, −4 V, −6 V, −8V, −10 Vgoing from the top curve to the bottom curve).

FIG. 7 is a graph plotting transfer characteristics (Ids as a functionof gate voltage Vg) and gm for the CAVET with an aperture lengthL_(ap)=15 μm and gate to aperture overlap L_(go)=4 μm.

FIG. 8 is a graph plotting I_(d), as a function of V_(ds) for the CAVET,for DC and pulsed operation, showing no current collapse using a 80 μspulse, for Vgs=0 to −10V using a step of V_(step)=−2V (from top tobottom curve) and wherein L_(ap)=15 μm and L_(go)=4 μm.

FIG. 9 plots I_(ds) as a function of V_(ds) for the CAVET havingL_(go)=2 μm and L_(go)=4 μm, wherein V_(gs)=0 to −10 V (V_(gs)=0V, −2 V,−4V, −6V, −8 V, −10 V going from the top curve to the bottom curve) andL_(ap)=3 μm, and showing increased leakage current as L_(go) decreases.

FIG. 10 is a flowchart illustrating a method of fabricating a CAVETaccording to one or more embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Overview

A CAVET is a vertical device comprised of an n-type doped drift regionto hold voltage and a horizontal 2DEG to carry current flowing from thesource, horizontally under a planar gate, and then vertically to thedrain through an aperture. A fundamental part of a CAVET is a CBL, whichblocks current flow and causes any on-state device current to flowthrough the aperture. Previously, the CBL has been achieved by dopingin-situ during growth in a metal-organic chemical vapor deposition(MOCVD) reactor or by ion implantation. The present invention, however,describes a CAVET with an ammonia-based MBE-grown p-GaN layer as theCBL. Specifically, an embodiment of the present invention features anactive buried Mg-doped p-GaN layer for current blocking purposes in aCAVET. This structure is highly advantageous for high power switchingapplications and for any device that requires a buried active p-GaNlayer for its functionality.

In CAVETs which employ a p-type current blocking GaN layer for which thep-type dopants are ion-implanted, the resulting p-type CBL typically isnot an active p-type layer, since damage caused by the implant processresults in a lower barrier to electron flow. That is, the number ofholes in the layer is substantially less than that in an active p-typelayer having the same density of p-type dopants. Subsequently, thebarrier to electron flow through a non-active p-type CBL is smaller thanthat for an active p-type current blocking layer, resulting in higherleakage currents through the non-active p-type current blocking layer.For example, an active p-type CBL may have a barrier to electron flowthrough the layer that is at least 2 or at least 3 electron-Volts (eV).Many ion-implanted CBLs, or CBLs formed by doping GaN with dopants otherthan Mg, for example Fe-doped CBL's, have barriers to electron flow thatare less than 1 eV. As used herein, an active p-type CBL is one in whichthe hole concentration is sufficiently large such that the barrier toelectron flow through the layer is at least 1 eV. That is, the productof the p-type doping concentration and the percent of dopants which areactive (i.e., result in a hole to be present in the valence band) issufficiently large to ensure that the barrier to electron flow is atleast 1 eV. In p-type III-N layers that are moderately or heavilydamaged, for example III-N layers which are ion-implanted with Mg, orMg-doped III-N layers that are passivated with hydrogen, for exampleMOCVD grown Mg-doped III-N layers, the hole concentration is typicallyrelatively small, and thus the resulting barrier to electron flow isless than 1 eV.

Technical Description

A base structure for the CAVET includes an n-type GaN (n-GaN) apertureregion grown on a thick, lightly doped, n-type drift region, which isetched back to the bottom n-GaN drift region using a mask to protect theaperture. On either side of the aperture is regrown p-GaN, which isregrown using an ammonia-assisted MBE technique. Thus, the CBLs areformed sandwiching the aperture region. The p-GaN layer is regrown in ahydrogen-free ambient and at low temperature, which ensures the activestate of the Mg dopants used for p-type doping of GaN, since the layeris neither heavily damaged (as with ion-implanted layers) or passivatedwith hydrogen (as with MOCVD grown Mg-doped III-N layers). The regrowthis performed with the aperture region protected by a mask (i.e., a maskover the aperture region) to ensure no p-type regrowth takes place ontop of the aperture region. Subsequently, the mask is etched away andthe surface is planarized, if needed. The device structure is completedby another regrowth of AlGaN/GaN channels to form the 2DEG.

Alternatively, the device can also be fabricated by creating theaperture by first growing a uniform p-type layer using theammonia-assisted MBE technique. The aperture region is etched and ann-type current carrying aperture is regrown, followed by the AlGaN/GaNchannel to form the 2DEG. The p-type layer thickness can range from 10nm to about 5 microns, as needed by the device functionality, with atypical layer being in the range of about 100 to 500 nanometers. Thickerlayers may be possible, but may complicate the fabrication process.

FIGS. 2( a)-(h) are schematic illustrations of the process flow used tofabricate a CAVET according to one embodiment of the present invention.

FIG. 2( a) represents a base structure 200, including n⁺-GaN 202, n⁻-GaN204, and n-GaN 206 layers, with the aperture layer being the n-GaN layer206, all of which layers are grown using metalorganic chemical vapordeposition (MOCVD). In one example, the n⁻-GaN layer 204 can be 6micrometers thick, doped with Silicon to a doping concentration of2×10¹⁶/cm³, and/or the n⁺-GaN 202 can be an n⁺-GaN substrate.

FIG. 2( b) represents the aperture being masked, wherein the regrowthmask 208 can be, for example, metal, Aluminum Nitride (AlN), or SiliconDioxide (SiO₂).

FIG. 2( c) represents the aperture region 210 of the n-GaN layer 206being left intact under the mask 208, while the rest of the n-GaN layer206 is etched away.

FIG. 2( d) represents the Mg-doped p-GaN 212 being grown using theammonia-based MBE technique to form the CBLs. In one example, the p-GaNlayer was regrown at a low temperature (840° C.), was active, and neededno further activation [5], and the regrowth was done with the AlN layermasking the aperture 210 to prevent any regrowth on the aperture region.Although a layer of p-GaN 212 is shown in FIG. 2( d) to be depositedover the regrowth mask, in some implementations the composition of theregrowth mask and the regrowth conditions are selected such thatsubstantially no p-GaN is regrown over the regrowth mask.

FIG. 2( e) represents the removal of the mask 208 and the planarizationof the surface 214, if necessary. The mask can be etched away using KOH,for example.

FIG. 2( f) represents regrowth by ammonia MBE, of the AlGaN/GaN layers216, 218 to form the channel with the 2DEG. For example, the step cancomprise the regrowth of a GaN layer 216 (e.g., UID GaN) and anAl_(0.3)Ga_(0.7)N layer 218 using the ammonia-based MBE technique.

FIG. 2( g) represents Si implants 220 a, 220 b being made for the sourcecontacts 222 into the Al_(0.3)Ga_(0.7)N layer 218 and the GaN layer 216,followed by an MOCVD activation anneal. The drain contact 224 (see FIG.2( i)) can be formed at the back using Ti/Au/Ni contact, for example.

FIG. 2( h) represents deposition of the gate dielectric 226 by, e.g.,atomic layer deposition (ALD), and the deposition of the source metals222 (e.g, Ti/Au/Ni).

The end result of this process flow is a CAVET with an ammonia-based MBEregrown active buried p-type layer 212, as illustrated in FIG. 2( i).

FIG. 2( i) illustrates a CAVET 228 including a channel region 230, theaperture region 210 (e.g., n-type GaN) sandwiched between theIII-nitride CBL 212, the drift region 204; and the gate 232. The channel230 can comprise a 2DEG confined in a GaN layer 216 by an AlGaN barrierlayer 218 on or above the GaN layer 216. An n-type doping concentrationin the drift region 204 can be less than an n-type doping concentrationin the aperture region 210. The gate 232 can comprise a Ni/Au/Ni gate232 deposited over ALD deposited Al₂O₃ gate dielectric 224. In FIG. 2(i), the CBL 212 and the source 222 are electrically connected 236 suchthat there is no bias across any part of the source 222 and the CBL 212.The electrical connection can be formed by etching a trench through tothe p-GaN layer 212 prior to deposition of the source metal, and thendepositing the source metal in the trench as well as over the Siimplanted regions 220 a and 220 b.

Depending on the growth parameters for the CBL, as well as conditionsand parameters used for growth and deposition of subsequent devicelayers, the CBL may have a barrier to electron flow that is at least 1eV, at least 2 eV, or at least 3 eV. While a 1 eV barrier may besufficient for device operation at lower voltages, for examplesource-drain voltages of less than 100V, a larger barrier, such as atleast 2 eV or at least 3 eV, may be preferable for operation at highervoltages, such as greater than 300V or greater than 600V.

FIG. 3 shows a band diagram of a structure wherein a conduction bandE_(c) offset and valence band E_(F) offset between the CBL and theenergy of the electrons and holes, respectively, in the 2DEG (e.g.,Fermi level, E_(F)), can be at least 3 eV. In FIG. 3, the CBL 212 is ap-GaN layer with a doping level of 5×10¹⁹ cm⁻², the GaN channel layer216 is UID GaN, the drift region 204 comprises n-GaN with 2×10¹⁶ cm⁻³doping. The AlGaN 218 over the channel layer 216 is also shown.

Characterization

SIMS done on the CAVET structure of FIG. 2( i) showed a very wellbehaved (e.g., sharp) doping profile, as shown in FIG. 4( a).

The blocking capacity of the p-layer (CBL, 212) in the CAVET of FIG. 2(i) can be measured.

FIG. 4( b) plots the current density as a function of V_(ds) through ap-GaN CBL of a zero aperture CAVET.

In addition, the blocking capacity of the p-layer can be verifiedseparately by fabricating an n-p-n structure on a GaN substrate 500(e.g, n⁺-GaN with 3×10¹⁸ cm⁻³ doping), as shown in FIG. 5( a).

The structure of FIG. 5( a) comprises a p-GaN layer 502 (e.g., 100 nmthick, with 8×10¹⁹ cm⁻³ doping) between an n-GaN layer 504 (e.g., 6 μmthick with 2×10¹⁶ cm⁻³ doping) and an n⁺-GaN layer 506 (e.g., 120 nmthick with 3×10¹⁸ cm³ doping). This structure is contacted with contacts508, 510, e.g., Ti/Au (30/250 nm) contacts. The layers 506 and 502 wereregrown by ammonia Molecular Beam Epitaxy (MBE).

FIG. 5( b) shows the forward bias I-V, and FIG. 5( c) shows the reversebias I-V characteristic, measured between contacts 508, 510 of thestructure in FIG. 5( a). The leakage current of the p-n diode formed bylayers 504 and 502 was 164 μA at 528 V in reverse bias, with a peakfield of E_(peak)=176 V/μm and a breakdown voltage of 528 V.

Another diode having a similar structure to the one shown in FIG. 5( a)has a leakage current of 140 μA or 0.35 A/cm² at 400V in reverse bias,as shown in FIG. 5( d). FIG. 5( d) is a graph of the I-V characteristicsof the n/p/n structure on the GaN substrate, with the back p/n diode 512(comprising layers 504 and 502) under reverse bias showing 140 μAcurrent (and a current density of less than 0.4 A/cm²) at 400V appliedacross the back p/n diode 512. As such, the CAVET of FIG. 2( i) havingsuch a CBL, and lacking any other off-state leakage current paths, wouldhave a current density of less than 0.4 A/cm² flowing through the CBLwhen the device is biased in the off state with a source-drain voltageof 400V or less.

FIGS. 6-9 are measurements performed on the CAVET having the structureof FIG. 2( i).

The fabricated CAVET device with active p-CBL exhibits good transistorcharacteristics, with good channel modulation and a pinchoff of −10V, asshown in FIG. 6. FIG. 6 is a graph of the DC I-V characteristics of theCAVET, wherein the aperture's 210 length L_(ap)=15 μm, V_(gate)=0 to−10V, and V_(step)=−2V.

FIG. 7 shows a 75 μm wide CAVET device 228 with a 15 μm aperture 210length, registered a current of 3.6 kA/cm², where the active area of 30μm×75 μm was measured from source implant region 220 a to source implantregion 220 b (including the implant opening). A low on resistance(R_(on)) of 1.22 mΩ cm² was obtained from this device. A peaktransconductance (gm) of 148 mS/mm (of the source) was obtained from thetransfer characteristics of the device, as shown in FIG. 7.

Pulsed I_(DS)-V_(DS) characteristics, measured with gate pulsed at 80 μspulse width, showed no current collapse, as shown in FIG. 8.

With decreasing L_(go) (the gate-aperture overlap), the leakage currentincreased due to unmodulated electrons flowing from source through theaperture to the drain, as shown in FIG. 9.

Process Steps

FIG. 10 illustrates an example of a method of forming or fabricating aCAVET including an aperture region in a III-nitride CBL, wherein abarrier to electron flow through the III-nitride CBL is, for example, atleast 1 electron-Volt. The method can comprise one or more of thefollowing steps.

Block 1000 represents obtaining, growing, or forming a drift region(e.g., n⁻ GaN). The drift region can be formed on or above an n⁺-typeGaN substrate, for example.

Block 1002 represents forming an aperture region comprised of a firstIII-nitride layer. The first III-nitride layer can be an n-typeIII-nitride or n-type GaN layer 206, e.g., formed on the drift region.

The step can comprise defining an aperture region and a sacrificialregion in the first III-nitride layer (e.g., by forming a mask over theaperture region) prior to removing (e.g., etching) the first III-nitridelayer in the sacrificial region. The first III-nitride layer remainingafter removal of the sacrificial region can be the aperture region.Then, a III-nitride CBL (e.g., p-type III-nitride) can be formed aroundor on either side of the aperture region, e.g., in areas where the firstIII-nitride layer was removed.

Alternatively, a uniform p-type layer can be grown using theammonia-assisted MBE technique, on the drift region of the CAVET. Theaperture region can then be etched in the p-type layer. Then, an n-typecurrent carrying aperture region can be regrown in the etched apertureformed in the p-type layer.

The p-type layer thickness can have a thickness of 10 nm or more (forexample), as needed for device functionality. The current blockinglayer's thickness (e.g., at least 10 nanometers), hole concentration,and composition, can be such that the barrier to electron flow has thedesired value (e.g., at least 1 eV, at least 2 eV, or at least 3 eV, forexample).

The p-type III-nitride CBL can be grown with dopants and under growthconditions wherein the p-type III-nitride layer's dopants are activatedor the p-type III-nitride layer is active. The growth conditions caninclude a low temperature (e.g., at or below 900° C. or at 500-900° C.)and a hydrogen-free ambient, for example. The CBL can be grown usingammonia (NH₃) based molecular beam epitaxy (MBE).

The p-type current blocking layer can be grown by a Metal OrganicChemical Vapor Deposition (MOCVD) growth technique by doping the GalliumNitride layer with Mg dopants, and activated by annealing in a hydrogenfree environment at >700° C. to make the current blocking layer p-type.Then the top AlGaN/GaN layers (216, 218 in FIG. 2 or second III-nitridelayers in Block 1004) can be regrown in ammonia-MBE, which does notpassivate the Mg acceptors, thereby preserving the p-type behavior ofthe current blocking layer.

Block 1004 represents growing and fabricating subsequent devicefeatures, including a III-nitride active region or channel (and source,drain, gate) for the CAVET, on or above or below the p-type III-nitridelayer or CBL and the first III-nitride layer. The step can compriseforming one or more second III-nitride layers on both the firstIII-nitride layer and the III-nitride CBL. The III-nitride active regioncan comprise the second III-nitride layer. The mask 208 can be removedprior to forming the second III-nitride layer.

The growing and fabricating of subsequent device features can be underconditions wherein the p-type III-nitride layer's dopants remainactivated.

Block 1006 represents the end result, a III-nitride CAVET 228 asillustrated in FIG. 2( i). The CAVET 228 can include an aperture region210 in a III-nitride CBL 212, wherein a barrier to electron flow throughthe III-nitride CBL is at least 1 or 2 electron-Volt(s). The III-nitrideCBL can be an active p-type doped III-nitride layer 212 (e.g., activep-type GaN or active Magnesium doped layer), and/or have a holeconcentration greater than a similar p-type III-nitride layer doped byion implantation. The aperture region can comprise n-type GaN.

The CBL can cause on-state current to flow through the aperture region.

The device can further comprise an active region or channel 230comprising a 2DEG confined in a GaN layer 216 by an AlGaN barrier layer218; a source 222 contact to the GaN layer 216 and the AlGaN barrierlayer 218; a drift region 204, comprising one or more n-type GaN layers,wherein the CBL is between the drift region 204 and the active region orchannel 230; and a drain contact 224 to the drift region 204, wherein agate 232 is positioned on or above the active region or channel 230 andthe aperture 210, to modulate a current between the source and thedrain.

The n-type III-nitride drift region 204 is between the aperture region210 and the drain 224, An n-type doping concentration in the driftregion 204 can be less than an n-type doping concentration in theaperture region 210.

The source 222 and the CBL can be electrically connected 236 such thatthere is no bias across any part of the source and the CBL.

The III-nitride CAVET can comprise a CBL, wherein the CAVET is operableto prevent a current density of greater than 0.4 A/cm² from flowingthrough the CBL when the CAVET is biased in an off state with asource-drain voltage of about 400 V, or 400V or less (see also FIGS.3-9).

Advantages and Improvements

The present invention includes the following advantages and improvementsover the prior art:

1. An active buried Mg-doped GaN layer can be grown in situ without anyneed of an activation process.

2. The CBL is a homoepitaxial blocking layer.

3. The CAVET does not need implanted GaN as the CBL.

4. The CAVET provides an ability to collect any holes that are generatedduring the operation of the device, so as to increase devicereliability.

5. The method/device provides an effective manner to connect the sourceto the CBL, so that there is no bias across any part of the source andthe CBL, preventing electron injection from the source to the drain.

6. The method/device enables smooth high frequency switching because ofthe predictable response of the p-type CBL, as compared to CBLs createdusing implantation to create damage.

Another benefit to the present invention is the simplicity in processingof the device. The biggest challenge in a device like the CAVET is theCBL. The most cost-effective CBL is a p-GaN layer grown on top of then-drift region. When the p-n junction gets reverse biased during deviceoperation, it can hold a very large voltage, which is desirable for theworking of the device. The biggest challenge is to get an active buriedp-layer as subsequent AlGaN/GaN layers are grown on top to form the2DEG. However, under hydrogen ambient at high regrowth temperatures(˜1160° C.), a p-layer is not active. The present invention, on theother hand, ensures a buried active p-GaN layer in the structure, whichmakes it functional and more effective from a device performance pointof view.

Nomenclature

The terms “(AlInGaN)” “(In,Al)GaN”, or “GaN” as used herein (as well asthe terms “III-nitride,” “Group-III nitride”, or “nitride,” usedgenerally) refer to any alloy composition of the (Ga,Al,In,B)Nsemiconductors having the formula Ga_(w)Al_(x)In_(y)B_(z)N where 0≦w≦1,0≦x≦1, 0≦y≦1, 0≦z≦1, and w+x+y+z=1. These terms are intended to bebroadly construed to include respective nitrides of the single species,Ga, Al, In and B, as well as binary, ternary and quaternary compositionsof such Group III metal species. Accordingly, it will be appreciatedthat the discussion of the invention hereinafter in reference to GaN andAlGaN materials is applicable to the formation of various other(Ga,Al,In,B)N material species. Further, (Ga,Al,In,B)N materials withinthe scope of the invention may further include minor quantities ofdopants and/or other impurity or inclusional materials.

REFERENCES

The following references are incorporated by reference herein:

-   [1] S. Chowdhury et al., Presented at EMC 2008, Santa Barbara.-   [2] Srabanti Chowdhury, Brian L. Swenson and Umesh K. Mishra,    “Enhancement and Depletion Mode AlGaN/GaN CAVET With    Mg-Ion-Implanted GaN as Current Blocking Layer,” IEEE Electron    Device Letters, Vol. 29, No. 6, pp. 543-545, June 2008.-   [3] Srabanti Chowdhury, “AlGaN/GaN CAVETs for high power switching    application,” Ph.D. thesis, University of California Santa Barbara,    December 2010, including the following pages: cover, iii, viii-xiv,    and 154-155.-   [4] “p-n junctions on Ga-face GaN by NH3 molecular beam epitaxy with    low ideality factors and low reverse currents,” C. A. Hurni. et al,    Applied Physics Letters Vol. 97, 222113, November 2010.-   [5] S. Chowdhury et al, IEEE EDL, Vol. 29, 2008.-   [6] S. Chowdhury et al, DRC, South Bend, July 2010.

CONCLUSION

This concludes the description of the preferred embodiments of thepresent invention. The foregoing description of one or more embodimentsof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

What is claimed is:
 1. A method of fabricating a current aperturevertical electron transistor (CAVET), comprising: defining a currentaperture region and a sacrificial region in a first III-nitride layer;removing the first III-nitride layer in the sacrificial region; forminga III-nitride current blocking layer around the current aperture region;forming one or more second III-nitride layers on both the firstIII-nitride layer and the III-nitride current blocking layer; andelectrically connecting a source of the CAVET and the III-nitridecurrent blocking layer such that, during the CAVET's operation, there isno bias between the source and the III-nitride current blocking layer.2. The method of claim 1, wherein the III-nitride current blocking layeris grown using ammonia (NH₃) based molecular beam epitaxy (MBE).
 3. Themethod of claim 1, further comprising forming a mask over the currentaperture region prior to removing the first III-Nitride layer in thesacrificial region.
 4. The method of claim 3, further comprisingremoving the mask prior to forming the second III-Nitride layers.
 5. Themethod of claim 1, wherein: the III-nitride current blocking layer isgrown by a Metal Organic Chemical Vapor Deposition (MOCVD) growthtechnique by doping a Gallium Nitride layer with Mg dopants, theIII-nitride current blocking layer is activated by annealing in ahydrogen free environment at a temperature above 700° C. to make theIII-nitride current blocking layer a p-type III-nitride current blockinglayer, and the second III-nitride layers comprising AlGaN/GaN layers areregrown in ammonia-MBE which does not passivate the Mg acceptors,thereby preserving the p-type behavior of the III-nitride currentblocking layer.
 6. The method of claim 1, further comprising: growingand doping the III-nitride current blocking layer comprising a holeconcentration and composition wherein the III-nitride current blockinglayer provides a barrier to electron flow, vertically through theIII-nitride current blocking layer, in a range of 1-2 electron-Volts. 7.The method of claim 1, further comprising: growing and doping theIII-nitride current blocking layer comprising a hole concentration andcomposition wherein the III-nitride current blocking layer provides abarrier to electron flow, vertically through the III-nitride currentblocking layer, of at least 2 electron-Volt.
 8. The method of claim 1,further comprising growing and doping the III-nitride current blockinglayer to form an active p-type doped III-nitride layer.
 9. The method ofclaim 8, wherein the III-nitride current blocking layer is an activep-type GaN layer.
 10. The method of claim 9, further comprising dopingthe III-nitride current blocking layer with Magnesium to form the p-typeGaN layer comprising an active Magnesium doped layer.
 11. The method ofclaim 1, wherein the current aperture region comprises n-type GaN. 12.The method of claim 1, further comprising: forming a drain; and formingan n-type III-nitride drift region between the current aperture regionand the drain, wherein an n-type doping concentration in the n-typeIII-nitride drift region is less than an n-type doping concentration inthe current aperture region.
 13. The method of claim 1, furthercomprising: forming an active region or channel comprising a twodimensional electron gas confined in a GaN layer by an AlGaN barrierlayer; depositing the source comprising a source contact to the GaNlayer and the AlGaN barrier layer; forming a drift region, comprisingone or more n-type GaN layers, wherein the III-nitride current blockinglayer is between the drift region and the active region or channel;depositing a drain contact to the drift region; and positioning a gateon or above the active region or channel and the current apertureregion, to modulate a current between the source contact and the draincontact.
 14. The method of claim 13, further comprising: selecting alateral position of the gate above the channel and the current apertureregion.
 15. The method of claim 14, further comprising: selecting alateral extension of the gate over the channel and the current apertureregion to control a peak electric field in the CAVET; and providing adielectric layer between the lateral extension and the channel.
 16. Themethod of claim 1, wherein the current blocking layer causes on-statecurrent to flow through the current aperture region.
 17. The method ofclaim 1, wherein the III-nitride current blocking layer is such that theCAVET is operable to prevent a current density of greater than 0.4 A/cm²from flowing through the current blocking layer when the CAVET is biasedin an off state with a source-drain voltage of 400V.
 18. The method ofclaim 1, wherein the III-nitride current blocking layer surrounds and isall around the current aperture region.
 19. The method of claim 1,further comprising: forming a mask over the current aperture region;removing the first III-Nitride layer in the sacrificial region; andgrowing a p-type Gallium Nitride (GaN) layer by Metal Organic ChemicalVapor Deposition (MOCVD) and selecting growth conditions wherein nop-type GaN is grown over the mask and the III-nitride current blockinglayer, comprising the p-type GaN around the current aperture region, isformed.